Stack is quite simple. Many computer systems have stacks built into their circuitry. They also have machine-level instructions to operate the hardware stack. Stack is ______ in computer systems.

A．useless

B．not important

C．simple but important

D．too simple to be useful

超外差式接收机利用（）两个器件把高频回波信号变换成便于滤波和处理的中频信号。